DetNet N. Finn
Internet-Draft Huawei Technologies Co. Ltd
Intended status: Standards Track J-Y. Le Boudec
Expires: April 25, 2019 E. Mohammadpour
EPFL
J. Zhang
Huawei Technologies Co. Ltd
B. Varga
J. Farkas
Ericsson
October 22, 2018
DetNet Bounded Latency
draft-finn-detnet-bounded-latency-02
Abstract
This document presents a parameterized timing model for Deterministic
Networking (DetNet), so that existing and future standards can
achieve the DetNet quality of service features of bounded latency and
zero congestion loss. It defines requirements for resource
reservation protocols or servers. It calls out queuing mechanisms,
defined in other documents, that can provide the DetNet quality of
service.
Status of This Memo
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Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 4
3. Terminology and Definitions . . . . . . . . . . . . . . . . . 4
4. DetNet bounded latency model . . . . . . . . . . . . . . . . 4
4.1. Flow creation . . . . . . . . . . . . . . . . . . . . . . 4
4.2. Relay system model . . . . . . . . . . . . . . . . . . . 5
5. Computing End-to-end Latency Bounds . . . . . . . . . . . . . 7
5.1. Non-queuing delay bound . . . . . . . . . . . . . . . . . 7
5.2. Queuing delay bound . . . . . . . . . . . . . . . . . . . 8
5.2.1. Per-flow queuing mechanisms . . . . . . . . . . . . . 8
5.2.2. Per-class queuing mechanisms . . . . . . . . . . . . 8
6. Achieving zero congestion loss . . . . . . . . . . . . . . . 10
6.1. A General Formula . . . . . . . . . . . . . . . . . . . . 10
7. Queuing model . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Queuing data model . . . . . . . . . . . . . . . . . . . 11
7.2. Preemption . . . . . . . . . . . . . . . . . . . . . . . 13
7.3. Time-scheduled queuing . . . . . . . . . . . . . . . . . 13
7.4. Time-Sensitive Networking with Asynchronous Traffic
Shaping . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.5. IntServ . . . . . . . . . . . . . . . . . . . . . . . . . 15
8. Time-based DetNet QoS . . . . . . . . . . . . . . . . . . . . 19
8.1. Cyclic Queuing and Forwarding . . . . . . . . . . . . . . 19
8.2. Time Scheduled Queuing . . . . . . . . . . . . . . . . . 19
9. Parameters for the bounded latency model . . . . . . . . . . 20
9.1. Sender parameters . . . . . . . . . . . . . . . . . . . . 20
9.2. Relay system parameters . . . . . . . . . . . . . . . . . 21
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
10.1. Normative References . . . . . . . . . . . . . . . . . . 21
10.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
The ability for IETF Deterministic Networking (DetNet) or IEEE 802.1
Time-Sensitive Networking (TSN, [IEEE8021TSN]) to provide the DetNet
services of bounded latency and zero congestion loss depends upon A)
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configuring and allocating network resources for the exclusive use of
DetNet/TSN flows; B) identifying, in the data plane, the resources to
be utilized by any given packet, and C) the detailed behavior of
those resources, especially transmission queue selection, so that
latency bounds can be reliably assured. Thus, DetNet is an example
of an INTSERV Guaranteed Quality of Service [RFC2212]
As explained in [I-D.ietf-detnet-architecture], DetNet flows are
characterized by 1) a maximum bandwidth, guaranteed either by the
transmitter or by strict input metering; and 2) a requirement for a
guaranteed worst-case end-to-end latency. That latency guarantee, in
turn, provides the opportunity for the network to supply enough
buffer space to guarantee zero congestion loss. To be of use to the
applications identified in [I-D.ietf-detnet-use-cases], it must be
possible to calculate, before the transmission of a DetNet flow
commences, both the worst-case end-to-end network latency, and the
amount of buffer space required at each hop to ensure against
congestion loss.
This document references specific queuing mechanisms, defined in
other documents, that can be used to control packet transmission at
each output port and achieve the DetNet qualities of service. This
document presents a timing model for sources, destinations, and the
network nodes that relay packets that is applicable to all of those
referenced queuing mechanisms. The parameters specified in this
model:
o Characterize a DetNet flow in a way that provides externally
measurable verification that the sender is conforming to its
promised maximum, can be implemented reasonably easily by a
sending device, and does not require excessive over-allocation of
resources by the network.
o Enable reasonably accurate computation of worst-case end-to-end
latency, in a way that requires as little detailed knowledge as
possible of the behavior of the Quality of Service (QoS)
algorithms implemented in each device, including queuing, shaping,
metering, policing, and transmission selection techniques.
Using the model presented in this document, it should be possible for
an implementor, user, or standards development organization to select
a particular set of queuing mechanisms for each device in a DetNet
network, and to select a resource reservation algorithm for that
network, so that those elements can work together to provide the
DetNet service.
This document does not specify any resource reservation protocol or
server. It does not describe all of the requirements for that
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protocol or server. It does describe requirements for such resource
reservation methods, and for queuing mechanisms that, if met, will
enable them to work together.
NOTE: This draft is not yet complete, but it is sufficiently so to
share with the Working Group and to obtain opinions and direction.
The present intent of is for this draft to become a normative RFC,
defining how one SHALL/SHOULD provide the DetNet quality of service.
There are still a few authors' notes to each other present in this
draft.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The lowercase forms with an initial capital "Must", "Must Not",
"Shall", "Shall Not", "Should", "Should Not", "May", and "Optional"
in this document are to be interpreted in the sense defined in
[RFC2119], but are used where the normative behavior is defined in
documents published by SDOs other than the IETF.
3. Terminology and Definitions
This document uses the terms defined in
[I-D.ietf-detnet-architecture].
4. DetNet bounded latency model
4.1. Flow creation
The bounded latency model assumes the use of the following paradigm
for provisioning a particular DetNet flow:
1. Perform any configuration required by the relay systems in the
network for the classes of service to be offered, including one
or more classes of DetNet service. This configuration is not
tied to any particular flow.
2. Characterize the DetNet flow in terms of limitations on the
sender [Section 9.1] and flow requirements [Section 9.2].
3. Establish the path that the DetNet flow will take through the
network from the source to the destination(s). This can be a
point-to-point or a point-to-multipoint path.
4. Select one of the DetNet classes of service for the DetNet flow.
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5. Compute the worst-case end-to-end latency for the DetNet flow.
In the process, determine whether sufficient resources are
available for that flow to guarantee the required latency and to
provide zero congestion loss.
6. Assuming that the resources are available, commit those resources
to the flow. This may or may not require adjusting the
parameters that control the queuing mechanisms at each hop along
the flow's path.
This paradigm can be static and/or dynamic, and can be implemented
using peer-to-peer protocols or using a central server model. In
some situations, backtracking and recursing through this list may be
necessary.
Issues such as un-provisioning a DetNet flow in favor of another when
resources are scarce are not considered. How the path to be taken by
a DetNet flow is chosen is not considered in this document.
4.2. Relay system model
In Figure 1 we see a breakdown of the per-hop latency experienced by
a packet passing through a relay system, in terms that are suitable
for computing both hop-by-hop latency and per-hop buffer
requirements.
DetNet relay node A DetNet relay node B
+-------------------------+ +------------------------+
| Queuing | | Queuing |
| Regulator subsystem | | Regulator subsystem |
| +-+-+-+-+ +-+-+-+-+ | | +-+-+-+-+ +-+-+-+-+ |
-->+ | | | | | | | | | + +------>+ | | | | | | | | | + +--->
| +-+-+-+-+ +-+-+-+-+ | | +-+-+-+-+ +-+-+-+-+ |
| | | |
+-------------------------+ +------------------------+
||||||||||
Internet-Draft DetNet Bounded Latency October 2018
[2] or the processing delay [4].) There are six delays that a packet
can experience from hop to hop.
1. Output delay
The time taken from the selection of a packet for output from a
queue to the transmission of the first bit of the packet on the
physical link. If the queue is directly attached to the physical
port, output delay can be a constant. But, in many
implementations, the queuing mechanism in a forwarding ASIC is
separated from a multi-port MAC/PHY, in a second ASIC, by a
multiplexed connection. This causes variations in the output
delay that are hard for the forwarding node to predict or control.
2. Link delay
The time taken from the transmission of the first bit of the
packet to the reception of the last bit, assuming that the
transmission is not suspended by a preemption event. This delay
has two components, the first-bit-out to first-bit-in delay and
the first-bit-in to last-bit-in delay that varies with packet
size. The former is typically measured by the Precision Time
Protocol and is constant (see [I-D.ietf-detnet-architecture]).
However, a virtual "link" could exhibit a variable link delay.
3. Preemption delay
If the packet is interrupted (e.g. [IEEE8023br] and [IEEE8021Qbu]
preemption) in order to transmit another packet or packets, an
arbitrary delay can result.
4. Processing delay
This delay covers the time from the reception of the last bit of
the packet to the time the packet is enqueued in the regulator
(Queuing subsystem, if there is no regulation). This delay can be
variable, and depends on the details of the operation of the
forwarding node.
5. Regulator delay
This is the time spent from the insertion of the last bit of a
packet into a regulation queue until the time the packet is
declared eligible according to its regulation constraints. We
assume that this time can be calculated based on the details of
regulation policy. If there is no regulation, this time is zero.
6. Queuing subsystem delay
This is the time spent for a packet from being declared eligible
until being selected for output on the next link. We assume that
this time is calculable based on the details of the queuing
mechanism. If there is no regulation, this time is from the
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insertion of the packet into a queue until it is selected for
output on the next link.
Not shown in Figure 1 are the other output queues that we presume are
also attached to that same output port as the queue shown, and
against which this shown queue competes for transmission
opportunities.
The initial and final measurement point in this analysis (that is,
the definition of a "hop") is the point at which a packet is selected
for output. In general, any queue selection method that is suitable
for use in a DetNet network includes a detailed specification as to
exactly when packets are selected for transmission. Any variations
in any of the delay times 1-4 result in a need for additional buffers
in the queue. If all delays 1-4 are constant, then any variation in
the time at which packets are inserted into a queue depends entirely
on the timing of packet selection in the previous node. If the
delays 1-4 are not constant, then additional buffers are required in
the queue to absorb these variations. Thus:
o Variations in output delay (1) require buffers to absorb that
variation in the next hop, so the output delay variations of the
previous hop (on each input port) must be known in order to
calculate the buffer space required on this hop.
o Variations in processing delay (4) require additional output
buffers in the queues of that same Detnet relay node. Depending
on the details of the queueing subsystem delay (6) calculations,
these variations need not be visible outside the DetNet relay
node.
5. Computing End-to-end Latency Bounds
5.1. Non-queuing delay bound
End-to-end latency bounds can be computed using the delay model in
Section 4.2. Here it is important to be aware that for several
queuing mechanisms, the worst-case end-to-end delay is less than the
sum of the per-hop worst-case delays. An end-to-end latency bound
for one DetNet flow can be computed as
end_to_end_latency_bound = non_queuing_latency + queuing_latency
The two terms in the above formula are computed as follows. First,
at the h-th hop along the path of this DetNet flow, obtain an upper
bound per-hop_non_queuing_latency[h] on the sum of delays 1,2,3,4 of
Figure 1. These upper-bounds are expected to depend on the specific
technology of the node at the h-th hop but not on the T-SPEC of this
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DetNet flow. Then set non_queuing_latency = the sum of per-
hop_non_queuing_latency[h] over all hops h.
5.2. Queuing delay bound
Second, compute queuing_latency as an upper bound to the sum of the
queuing delays along the path. The value of queuing_latency depends
on the T-SPEC of this flow and possibly of other flows in the
network, as well as the specifics of the queuing mechanisms deployed
along the path of this flow.
For several queuing mechanisms, queuing_latency is less than the sum
of upper bounds on the queuing delays (5,6) at every hop. This
occurs with (1) per-flow queuing, and (2) per-class queuing with
regulators, as explained in Section 5.2.1, Section 5.2.2, and
Section 7.
For other queuing mechanisms the only available value of
queuing_latency is the sum of the per-hop queuing delay bounds. In
such cases, the computation of per-hop queuing delay bounds must
account for the fact that the T-SPEC of a DetNet flow is no longer
satisfied at the ingress of a hop, since burstiness increases as one
flow traverses one DetNet node.
5.2.1. Per-flow queuing mechanisms
With such mechanisms, each flow uses a separate queue inside every
node. The service for each queue is abstracted with a guaranteed
rate and a delay. For every flow the per-node delay bound as well as
end-to-end delay bound can be computed from the traffic specification
of this flow at its source and from the values of rates and latencies
at all nodes along its path. Details of calculation for IntServ are
described in Section 7.5.
5.2.2. Per-class queuing mechanisms
With such mechanisms, the flows that have the same class share the
same queue. A practical example is the queuing mechanism in Time
Sensitive Networking. One key issue in this context is how to deal
with the burstiness cascade: individual flows that share a resource
dedicated to a class may see their burstiness increase, which may in
turn cause increased burstiness to other flows downstream of this
resource. Computing latency upper bounds for such cases is
difficult, and in some conditions impossible
[charny2000delay][bennett2002delay]. Also, when bounds are obtained,
they depend on the complete configuration, and must be recomputed
when one flow is added.
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A solution to deal with this issue is to reshape the flows at every
hop. This can be done with per-flow regulators (e.g. leaky bucket
shapers), but this requires per-flow queuing and defeats the purpose
of per-class queuing. An alternative is the interleaved regulator,
which reshapes individual flows without per-flow queuing
([Specht2016UBS], [IEEE8021Qcr]"). With an interleaved regulator,
the packet at the head of the queue is regulated based on its (flow)
regulation constraints; it is released at the earliest time at which
this is possible without violating the constraint. One key feature
of per-flow or interleaved regulator is that, it does not increase
worst-case latency bounds [le_boudec_theory_2018]. Specifically,
when an interleaved regulator is appended to a FIFO subsystem, it
does not increase the worst-case delay of the latter.
Figure 2 shows an example of a network with 5 nodes, per-class
queuing mechanism and interleaved regulators as in Figure 1. An end-
to-end delay bound for flow f, traversing nodes 1 to 5, is calculated
as follows:
end_to_end_latency_bound_of_flow_f = C12 + C23 + C34 + S4
In the above formula, Cij is a bound on the aggregate response time
of queuing subsystem in node i and interleaved regulator of node j,
and S4 is a bound on the response time of the queuing subsystem in
node 4 for flow f. In fact, using the delay definitions in
Section 4.2, Cij is a bound on sum of the delays 1,2,3,6 of node i
and 4,5 of node j. Similarly, S4 is a bound on sum of the delays
1,2,3,6 of node 4. A practical example of queuing model and delay
calculation is presented Section 7.4.
f
----------------------------->
+---+ +---+ +---+ +---+ +---+
| 1 |---| 2 |---| 3 |---| 4 |---| 5 |
+---+ +---+ +---+ +---+ +---+
\__C12_/\__C23_/\__C34_/\_S4_/
Figure 2: End-to-end latency computation example
REMARK: The end-to-end delay bound calculation provided here gives a
much better upper bound in comparison with end-to-end delay bound
computation by adding the delay bounds of each node in the path of a
flow [TSNwithATS].
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6. Achieving zero congestion loss
When the input rate to an output queue exceeds the output rate for a
sufficient length of time, the queue must overflow. This is
congestion loss, and this is what deterministic networking seeks to
avoid.
6.1. A General Formula
To avoid congestion losses, an upper bound on the backlog present in
the regulator and queuing subsystem of Figure 1 must be computed
during resource reservation. This bound depends on the set of flows
that use these queues, the details of the specific queuing mechanism
and an upper bound on the processing delay (4). The queue must
contain the packet in transmission plus all other packets that are
waiting to be selected for output.
A conservative backlog bound, that applies to all systems, can be
derived as follows.
The backlog bound is counted in data units (bytes, or words of
multiple bytes) that are relevant for buffer allocation. For every
class we need one buffer space for the packet in transmission, plus
space for the packets that are waiting to be selected for output.
Excluding transmission and preemption times, the packets are waiting
in the queue since reception of the last bit, for a duration equal to
the processing delay (4) plus the queuing delays (5,6).
Let
o nb_classes be the number of classes of traffic that may use this
output port
o total_in_rate be the sum of the line rates of all input ports that
send traffic of any class to this output port. The value of
total_in_rate is in data units (e.g. bytes) per second.
o nb_input_ports be the number input ports that send traffic of any
class to this output port
o max_packet_length be the maximum packet size for packets of any
class that may be sent to this output port. This is counted in
data units.
o max_delay45 be an upper bound, in seconds, on the sum of the
processing delay (4) and the queuing delays (5,6) for a packet of
any class at this ouput port.
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Then a bound on the backlog of traffic of all classes in the queue at
this output port is
backlog_bound = ( nb_classes + nb_input_ports ) *
max_packet_length + total_in_rate* max_delay45
7. Queuing model
7.1. Queuing data model
Sophisticated queuing mechanisms are available in Layer 3 (L3, see,
e.g., [RFC7806] for an overview). In general, we assume that "Layer
3" queues, shapers, meters, etc., are precisely the "regulators"
shown in Figure 1. The "queuing subsystems" in this figure are not
the province solely of bridges; they are an essential part of any
DetNet relay node. As illustrated by numerous implementation
examples, some of the "Layer 3" mechanisms described in documents
such as [RFC7806] are often integrated, in an implementation, with
the "Layer 2" mechanisms also implemented in the same system. An
integrated model is needed in order to successfully predict the
interactions among the different queuing mechanisms needed in a
network carrying both DetNet flows and non-DetNet flows.
Figure 3 shows the general model for the flow of packets through the
queues of a DetNet relay node. Packets are assigned to a class of
service. The classes of service are mapped to some number of
regulator queues. Only DetNet/TSN packets pass through regulators.
Queues compete for the selection of packets to be passed to queues in
the queuing subsystem. Packets again are selected for output from
the queuing subsystem.
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|
+--------------------------------V----------------------------------+
| Class of Service Assignment |
+--+------+----------+---------+-----------+-----+-------+-------+--+
| | | | | | | |
+--V-+ +--V-+ +--V--+ +--V--+ +--V--+ | | |
|Flow| |Flow| |Flow | |Flow | |Flow | | | |
| 0 | | 1 | ... | i | | i+1 | ... | n | | | |
| reg| | reg| | reg | | reg | | reg | | | |
+--+-+ +--+-+ +--+--+ +--+--+ +--+--+ | | |
| | | | | | | |
+--V------V----------V--+ +--V-----------V--+ | | |
| Trans. selection | | Trans. select. | | | |
+----------+------------+ +-----+-----------+ | | |
| | | | |
+--V--+ +--V--+ +--V--+ +--V--+ +--V--+
| out | | out | | out | | out | | out |
|queue| |queue| |queue| |queue| |queue|
| 1 | | 2 | | 3 | | 4 | | 5 |
+--+--+ +--+--+ +--+--+ +--+--+ +--+--+
| | | | |
+----------V----------------------V--------------V-------V-------V--+
| Transmission selection |
+----------+----------------------+--------------+-------+-------+--+
| | | | |
V V V V V
DetNet/TSN queue DetNet/TSN queue non-DetNet/TSN queues
Figure 3: IEEE 802.1Q Queuing Model: Data flow
Some relevant mechanisms are hidden in this figure, and are performed
in the queue boxes:
o Discarding packets because a queue is full.
o Discarding packets marked "yellow" by a metering function, in
preference to discarding "green" packets.
Ideally, neither of these actions are performed on DetNet packets.
Full queues for DetNet packets should occur only when a flow is
misbehaving, and the DetNet QoS does not include "yellow" service for
packets in excess of committed rate.
The Class of Service Assignment function can be quite complex, even
in a bridge [IEEE8021Q], since the introduction of [IEEE802.1Qci].
In addition to the Layer 2 priority expressed in the 802.1Q VLAN tag,
a DetNet relay node can utilize any of the following information to
assign a packet to a particular class of service (queue):
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o Input port.
o Selector based on a rotating schedule that starts at regular,
time-synchronized intervals and has nanosecond precision.
o MAC addresses, VLAN ID, IP addresses, Layer 4 port numbers, DSCP.
([I-D.ietf-detnet-dp-sol-ip], [I-D.ietf-detnet-dp-sol-mpls]) (Work
items are expected to add MPC and other indicators.)
o The Class of Service Assignment function can contain metering and
policing functions.
o MPLS and/or pseudowire ([RFC6658]) labels.
The "Transmission selection" function decides which queue is to
transfer its oldest packet to the output port when a transmission
opportunity arises.
7.2. Preemption
In IEEE Std 802.1Q, preemption is modeled as consisting of two MAC/
PHY stacks, one for packets that can be interrupted, and one for
packets that can interrupt the interruptible packets. The Class of
Service (queue) determines which packets are which. Only one layer
of preemption is supported. DetNet flows pass through the
interrupting MAC. Only best-effort queues pass through the
interruptible MAC, and can thus be preempted.
7.3. Time-scheduled queuing
In [IEEE8021Qbv], the notion of time-scheduling queue gates were
introduced. On below every output queue (the lower row of queues in
Figure 3) is a gate that permits or denies the queue to present data
for transmission selection. The gates are controlled by a rotating
schedule that can be locked to a clock that is synchronized with
other relay nodes. The DetNet class of service can be supplied by
queuing mechanisms based on time, rather than the regulator model in
Figure 3. These queuing mechanisms are discussed in Section 8,
below.
7.4. Time-Sensitive Networking with Asynchronous Traffic Shaping
Consider a network with a set of nodes (switches and hosts) along
with a set of flows between hosts. Hosts are sources or destinations
of flows. There are four types of flows, namely, control-data
traffic (CDT), class A, class B, and best effort (BE) in decreasing
order of priority. Flows of classes A and B are together referred to
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as AVB flows. It is assumed a subset of TSN functions as described
next.
It is also assumed that contention occurs only at the output port of
a TSN node. Each node output port performs per-class scheduling with
eight classes: one for CDT, one for class A traffic, one for class B
traffic, and five for BE traffic denoted as BE0-BE4 (according to TSN
standard). In addition, each node output port also performs per-flow
regulation for AVB flows using an interleaved regulator (IR), called
Asynchronous Traffic Shaper (ATS) in TSN. Thus, at each output port
of a node, there is one interleaved regulator per-input port and per-
class. The detailed picture of scheduling and regulation
architecture at a node output port is given by Figure 4. The packets
received at a node input port for a given class are enqueued in the
respective interleaved regulator at the output port. Then, the
packets from all the flows, including CDT and BE flows, are enqueued
in a class based FIFO system (CBFS) [TSNwithATS].
+--+ +--+ +--+ +--+
| | | | | | | |
|IR| |IR| |IR| |IR|
| | | | | | | |
+-++XXX++-+ +-++XXX++-+
| | | |
| | | |
+---+ +-v-XXX-v-+ +-v-XXX-v-+ +-----+ +-----+ +-----+ +-----+ +-----+
| | | | | | |Class| |Class| |Class| |Class| |Class|
|CDT| | Class A | | Class B | | BE4 | | BE3 | | BE2 | | BE1 | | BE0 |
| | | | | | | | | | | | | | | |
+-+-+ +----+----+ +----+----+ +--+--+ +--+--+ +--+--+ +--+--+ +--+--+
| | | | | | | |
| +-v-+ +-v-+ | | | | |
| |CBS| |CBS| | | | | |
| +-+-+ +-+-+ | | | | |
| | | | | | | |
+-v--------v-----------v---------v-------V-------v-------v-------v--+
| Strict Priority selection |
+--------------------------------+----------------------------------+
|
V
Figure 4: Architecture of a TSN node output port with interleaved
regulators (IRs)
The CBFS includes two CBS subsystems, one for each class A and B.
The CBS serves a packet from a class according to the available
credit for that class. The credit for each class A or B increases
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based on the idle slope, and decreases based on the send slope, both
of which are parameters of the CBS. The CDT and BE0-BE4 flows in the
CBFS are served by separate FIFO subsystems. Then, packets from all
flows are served by a transmission selection subsystem that serves
packets from each class based on its priority. All subsystems are
non-preemptive. Guarantees for AVB traffic can be provided only if
CDT traffic is bounded; it is assumed that the CDT traffic has an
affine arrival curve r t + b in each node, i.e. the amount of bits
entering a node within a time interval t is bounded by r t + b.
[[ EM: THE FOLLOWING PARAGRAPH SHOULD BE ALIGNED WITH Section 9.2. ]]
Additionally, it is assumed that flows are regulated at their source,
according to either leaky bucket (LB) or length rate quotient (LRQ).
The LB-type regulation forces flow f to conform to an arrival curve
r_f t+b_f . The LRQ-type regulation with rate r_f ensures that the
time separation between two consecutive packets of sizes l_n and
l_n+1 is at least l_n/r_f. Note that if flow f is LRQ-regulated, it
satisfies an arrival curve constraint r_f t + L_f where L_f is its
maximum packet size (but the converse may not hold). For an LRQ
regulated flow, b_f = L_f. At the source hosts, the traffic
satisfies its regulation constraint, i.e. the delay due to
interleaved regulator at hosts is ignored.
At each switch implementing an interleaved regulator, packets of
multiple flows are processed in one FIFO queue; the packet at the
head of the queue is regulated based on its regulation constraints;
it is released at the earliest time at which this is possible without
violating the constraint. The regulation type and parameters for a
flow are the same at its source and at all switches along its path.
Details of end-to-end delay bound calculation in such a system is
described in [TSNwithATS].
7.5. IntServ
In this section, a worst-case queuing latency calculating method is
provided. In deterministic network, the traffic of a flow is
constrained by arrival curve. Queuing mechanisms in a DetNet node
can be characterized and constrained by service curve. By using
arrival curve and service curve with Network Calculus theory
[NetCalBook], a tight worst-case queuing latency can be calculated.
Considering a DetNet flow at output port, R(s) is the cumulative
arrival data until time s. For any time period t, the incremental
arrival data is constrained by an arrival curve a(t)
R(s+t)-R(s) <= a(t), \any s>=0, t>=0
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The scheduling that a relay node performs to a DetNet flow can be
abstracted as service curve. It describes the minimal service the
network can offer. The service curve b(t) of a node is defined as
below, if the accumulative input data R and output data R_out of the
node satisfies
R_out(t) >= inf(R(s) + b(t-s) ), \any s <=t
where the operator "inf" calculates the greatest lower bound in
period t.
By calculating the maximum vertical deviation between arrival curve
a(t) and service curve b(t), one can obtain the backlog bound in data
unit
Backlog_bound = sup_t(a(t) - b(t) )
where operator "sup_t" calculates the minimum upper bound with
respect to t. The buffer space at a node should be no less than the
backlog bound to achieve zero congestion loss.
NOTE: Section 6.1 gives a general formula for computing the buffer
requirements. This is an alternative calculation based on the
arrival curve and service curve.
By calculating the maximum horizontal deviation between arrival curve
a(t) and service curve b(t), one can obtain the delay bound as below
Delay_bound = sup_s( inf_t( t>=0 | a(s) <= b(s+t) )
where the operator " inf_t" calculates the maximum lower bound with
respect to t, the operator "sup_s" calculates the minimum upper bound
with respect to s. Figure 5 shows an example of arival curve,
service curve, backlog bound h, and delay bound v.
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+ bit . *
| . *
| . *
| *
| * .
| * .
| * | . .. Service curve
*-----h-|---. ** Arrival curve
| v . h Delay_bound
| | . v Backlog_bound
| |.
+-------.--------------------+ time
Figure 5: Computation of backlog bound and delay bound. Note that
arrival and service curves are not necessary to be linear.
Note that in the formula of Delay_bound, the service curve b(t) can
describe either per-hop scheduling that a DetNet node offers to a
flow, or concatenation of multiple nodes that represents end-to-end
scheduling that DetNet path offers to a flow. In the latter case,
the obtained delay bound is end-to-end worst case delay. To
calculate this, we should at first derive the concatenated service
curve.
Consider a flow traverse two DetNet nodes, which offer service curve
b1(t) and b2(t) sequentially. Then concatenation of the two nodes
offers a service curve b_concatenated as below
b_concatenated(t) =inf_s (b1(s) + b2(t-s) ) , \any 0 <=s <=t
The concatenation of service curve can be directly generalized to
include more than two nodes.
a_out(t) = sup_u( a(t+u) - b(u) ), \any u>=0
In DetNet, the arrival curve and service curve can be characterized
by a group of parameters, which will be defined in Section 8.
Integrated service (IntServ) is an architecture that specifies the
elements to guarantee quality of service (QoS) on networks. To
satisfied guaranteed service, a flow must conform to a traffic
specification (T-spec), and reservation is made along a path, only if
routers are able to guarantee the required bandwidth and buffer.
Consider the traffic model which conforms to token bucket regulator
(r, b), with
o Token bucket depth (b).
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o Token bucket rate (r).
The traffic specification can be described as an arrival curve a(t)
alpha(t) = b + rt
This token bucket regulator requires that, during any time window of
width t, the number of bit for the flow is limited by alpha(t) = b +
rt.
If resource reservation on a path is applied, IntServ model on a
router can be described as a rate-latency service curve beta(t).
beta(t) = max(0, R(t-T))
It describes that bits might have to wait up to T before being served
with a rate greater or equal to R.
It should be noted that, the guaranteed service rate R is a share of
link's bandwidth. The choice of R is related to the specification of
flows which will transmit on this node. For example, in strict
priority policy, considering a flow with priority j, its share of
bandwidth may be R=c-sum(r_i), i